Joint arthroplasty is a well-known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. The orthopaedic prosthetic joint may include a number of components of various shapes and sizes. Some orthopaedic prosthetic components have a spherical shape or otherwise include a curved outer surface. For example, typical “ball-and-socket” prosthetic joints, such as hip and shoulder prosthetic joints, include a head component having a spherical shape, which is configured to be received in a corresponding bearing component which likewise has a spherical shape.
Recently, aspherical designs have been investigated. See, for example, U.S. Pat. No. 6,059,830. In such designs, the outer surface of the head component or the bearing component or both include an aspherical surface—i.e., include multiple arcuate surfaces with different radii.
Orthopaedic prosthetic components having spherical or aspherical outer surfaces are typically manufactured using a multi-step lathing process. Because a lathe is limited in its ability to form curved surfaces with a work piece held in a single orientation, several lathing process steps using different component orientations are required to form the spherical shape or other curved surface. This is compounded in the case of aspherical designs. Additionally, because typical lathes are unable to form curved surfaces within the tolerances required for orthopaedic applications, a subsequent grinding process is required to form the spherical shape or curved surface within the tolerances required.
In addition to lathing processes, other manufacturing processes may be used to fabricate orthopaedic prosthetic components. One such manufacturing process is electrical discharge machining (EDM). Electrical discharge machining may be used to machine materials that are electrically conductive. In electrical discharge machining, a potential difference is generated between an electrode, such as a wire electrode, of the electrical discharge machining tool and the work piece. The potential difference between the electrode and the work piece causes a spark to be generated. The spark erodes a portion of the work piece, and consecutive sparks between the electrode and work piece are used to remove material from the work piece. Because the electrode may also be damaged by the spark, the electrode is continuously replaced. For example, in electrical discharge machining using wire electrodes, the electrode wire is continuously advanced while the work piece is being fabricated.
The work piece may be shaped by moving the electrode and/or the work piece itself. Additionally, spherical shapes may be formed using electrical discharge machining by rotating the work piece while the electrode is moved along an arc. For example, a typical EDM includes a positioning motor to which the work piece is coupled. The positioning motor is used to slowly move the work piece while the wire electrode is moved along an arc. The adaptive feedback control used to cut the work piece is based, in part, on the speed and position of the work piece positioning motor. Typical positioning motors operate at about 1-2 revolutions per minute. At such rotational speeds, fabrication of spherical orthopaedic components using a typical electrical discharge machining process take about four to five days to complete.